Control system for induction motor



June 9, 1959 Filed Dec. 31, 1956 W. B. JARVINEN CONTROL SYSTEM FORINDUCTION MOTOR 2 Sheets-Sheet l Inventor: Willard. B. Jarvne'n,

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Willard B. Jarvinen,

CONTROL SYSTEM FOR INDUCTION MOTOR Willard B. JarVnen, Roanoke, Va.,assignor to General Electric Company, a corporation of New YorkApplication December 31, 1956, Serial No. 631,560

9 Claims. ('Cl. S18-225) This invention relates to control systems, moreparticularly for systems for controlling the operation of inductionmotors, and it h-as for an object the provision of a simple, reliableand improved control system of this character.

More specifically, the invention relates to `control systems for dualprimary winding induction motors, and a further object of the inventionis the provision of a control system for effecting precise and steplessspeed control of a motor of this type.

Another object of the invention is the provision of a stepless andprecise speed control of a dual primary winding inductor motorthroughout a wide range of speed and a full range of motoring andbraking torques.

A lstill further object of the invention is the provision of a controlsystem for a dual primary winding induction motor in which the speed ofthe motor is maintained approximately constant at any preset valuebetween 100% speed forward and 100% speed reverse throughout a fullrange of torques lfrom maximum motoring torque to maximum brakingtorque.

In carrying the invention into effect in one form thereof there isprovided an induction motor having dual primary windings. The twowindings are connected `to the power supply through independent sets ofsaturable reactors. Reversing switching means are provided forindependently connecting each IWinding to the power supply either forforward or for reverse rotation. A master switching device provides areference voltage of yadjustable value and means are provided forderiving from the motor a control voltage which is representative of itsspeed. The difference of the reference voltage and the speed voltage isutilized to control the reversing switching means to control therelative directions of the torques produced by the two primary windings.It is also used to control the saturable reactors `in circuit therewithto control the relative magnitudes of the two torques.

In order to illustrate the invention in an application lwhich imposeswidely varying operating conditions it is shown as embodied in a controlsystem of an induction motor for operating a bascule bridge. Some of thecontrol requirements for a motor to operate a bascule bridge are these:

(l) In opening the bridge the motor must start smoothly whatever the iceand snow load and accelerate `to full speed in a yrelatively short time,e.g. live to ten seconds. The torque should be built up Without shockand 200% or more of rated torque should be available to provide therequired rapid acceleration.

(2) At full load ltorque, the motor should run at 85 to 90 percent ofsynchronous speed, and it should also deliver 160% torque at 75 to 80percent speed. Normally the full speed operation may continue for about60 seconds until the bridge has completed 85 to 90 percent of itstravel.

(3) During the last l0 to l5 percent of its travel the motor should runat approximately of synchronous speed so that the bridge will proceedsmoothly into its United States Patent O 2,890,398 Patented June 9, 1959ICC fully open position and lock in without shock. During this periodthe leaf of the bridge presents a large area to the wind and theresulting load on the motor may exceed full load torque. To maintain thedesired speed in a cross-wind, therefore, the motor driving a leaf `of abascule bridge may be required to exert 150% braking torque.

(4) To close the bridge the same procedure is followed but with themotors operating in the reverse direction.

For a better and more complete understanding. of the invention referenceshould now be had to the following specifica-tion and to theaccompanying drawings of which Figs. 1A and 1B taken together and`'arranged with the top edge of Fig. 1B adjacent the lower edge of Fig.1A constitute a simple, diagrammatical sketch of an embodiment of theinvention, and Fig. 2 is a simple diagrammatic sketch of a single leafbascule bridge.

Referring now to the drawings, an induction motor 1 is employed tooperate the leaf 3 of a single leaf bascule bridge between the closedposition in which it is illustrated yand the open position which isindicated by dotted outline.

The motor 1 is mounted in la suitable position with respect to the leaf,preferably in the counterweight pit and its drive shaft is connectedthrough suitable reduction gearing to a rack arranged on thecounterweight end of the leaf.

The motor 1 is a polyphase-wound rotor induction motor of the typehaving dual primary windings. As shown in Fig. l lthe motor has twoidentical Y-connected primary windings 5 and 6 wound to provide the samenumber of poles. It also has a Y-connected second- -ary winding 7.Voltage of suitable magnitude is supplied to the primary windings from asuitable three-phase source such as the three supply conductors 8, 9 and10. For the purpose of controlling the directions of the torquesproduced by the windings 5 and 6 with respect to each other, suitablereversing switching means are included in the connections between eachprimary winding and the source. The reversing switching means for theprimary winding 5 are illustrated las electromagnetic contactors 11 and12 of which the contactor 11 is regarded as the forward contactor andcontactor 12 as the reverse contactor. Similar forward and reversecontactors 13 and 14 respectively are included in the circuit of primarywinding 6.

For the purpose of varying the magnitude of the cur'- rents supplied tothe primary windings 5 and 6, suitable controllable current limitingmeans are provided. The current limiting means for the primary winding 5are illustrated as three individual saturable reactors 15, 16 and 17having magnetic core members 15a, 16a and 17a respectively upon whichare mounted main load circuit reactance windings 15b, 16b and 17b andsaturation control windings 15c, 16C and 17e respectively. Each of themain reactance windings 15b, 16b and 17b is illustrated as comprisingtwo separate coils in parallel and each is connected in a differentphase of the primary circuit. Similarly connected in circuit with theseparate phases of primary winding 6 are the main load circuit reactancewindings 18b, 19h and 20b of individual saturable reactors 18, 19 and 20respectively. f

Comprising the secondary circuit of the motor are three resistors 21, 22,and 23 connected in delta and three reactors 24, 25 and 26 connected inan independent Y. The rating of the resistors and reactors depends uponthe rating of the motor. For' a motor having a rated current of 153amperes and a rated voltage per phase of 254 volts, each of thesecondary resistors might appropriately have a value of .8 ohm and eachof the secondary reactors might appropriately have a value of .8 ohm atvolts and 60 cycles.

For the purpose of varying the saturation of the reactors 15, 16 and 17their saturation control windings are connected to be supplied from theoutput of a suitable magnetic amplifier 27 of which the input circuit isconnected to be energized by an error voltage which is representative ofthe difference between the desired speed of the motor and its actualspeed. The magnetic amplifier 27 is of the type generally known in theart as self-saturating. It is also known as an amplistat. As Shown inFig. l it comprises a pair of output circuit reactance windings 28 and29 wound upon a magnetic core structure (not shown) and connected withrectifiers 30, 30a, 31 and 31a in a full wave rectifying bridge circuitwhich is supplied from a suitable source of alternating voltage such asthe supply conductors 33a and 34. Diagonally opposite bridge points 27aand 27b constitute the positive and negative output terminals. For thepurpose of controlling the saturation of its core, the magneticamplifier is provided with a negative bias coil 36 and a main saturationcontrol winding 37. To the bias coil 36 constant direct current ofsuitable magnitude is supplied from a suitable source such as the directvoltage supply conductors 38 and 39.

A similar magnetic amplifier 40 having a negative bias coil 41 and amain saturation control winding 42 supplies direct current of variablemagnitude to the saturation control windings 18C, 19C and 20c ofsaturable reactors 18, 19 and 20 which are connected in the circuit ofmotor primary winding 6. The main saturation control windings 37 and 42are connected to be energized by the previously mentioned error voltage.

A direct control voltage which is representative of the actual speed ofthe motor 1 is provided by means of a tachomcter generator 43 which ismechanically coupled to the shaft of motor 1. A reference voltage ofadjustable magnitude and reversible polarity which is representative ofthe desired speed and direction of rotation of the motor is provided bymeans of a reversing type master switch 44 and two potentiometers 45 and46 controlled thereby and energized from a suitable source of directvoltage such as supply conductors 38 and 39. The potentiometers and 46are serially connected across the supply conductors 38 and 39.Preferably they have equal resistances so that the voltage at theircommon terminal 45a is midway between the voltages of the supplyconductors.

The master switch 44 has a central off position and a plurality offorward and reverse positions for effecting forward and reverse rotationof the motor. Forward rotation raises the leaf 3 and reverse rotationlowers it. An interlock contact 44h which is normally closed in the oftposition of the master switch connects the output terminal 44a to thecommon terminal 45a. In the operating positions of the master switch theinterlock contact 44h is open and the output terminal 44a is connectedto different points on one or the other of the potentiomcters 4-5 and 46so that there is supplied to the output terminal a voltage of which thepolarity and magnitude with respect to the center terminal 45o dependsupon the direction and position to which the master switch is operated.The voltage between the terminal 45a and the output terminal 44a is thereference voltage.

In order to compare the actual speed of the motor with the desiredspeed, a voltage-comparison circuit 4S is provided to which are suppliedin opposing relationship the reference voltage and a selectable portionof the terminal voltage of the tachometer generator. An adjustablerheostat 49 connected across the terminals of the tachometer generatorand included in the comparison circuit provides the selectable amount oftachometer voltage which is compared with the reference voltage. Alsoincluded in the comparison circuit are the main saturation controlwindings 37 and 42 of magnetic amplifiers 27 and 40 respectively. Thecomparison circuit is readily traced from the output terminal 44athrough a reactor 50, control winding 42 through normally closedcontacts 5117 and 51e of a reversing contactor 51, saturation controlwinding 37 via normally closed contacts 52h and 52e of a secondreversing contactor 52, active portion of potentiometer 49 and thence byconductor 53 to the center terminal 45a.

Connected across the output terminals of magnetic ampliers 27 and 4) arethe operating coils 55a and 56a respectively, of voltage-sensitivecontrol relays 55 and 56. Although these relays may be of any suitabletype they are preferably general purpose D.C. operated instantaneouspick-up and dropout devices having dropout voltages which are relativelylow with respect to their pickup voltages. Typically they may have 4()volt pickup and 15 volt dropout. These relays have normally closedcontacts 55h and 56h and normally open contacts 55e and 56erespectively. As shown, the normally closed contacts of each areconnected in the operating coil circuit of the other to provideelectrical interlocking.

For the purpose of controlling the energization of the reversingcontactors 11 and 12 for motor primary winding 5 relay 57 is provided,and a similar relay 58 is provided for controlling the energization ofreversing contactors 13 and 14 for primary winding 6. The open atingcoil 57a of relay 57 and the operating coil 52a of reversing contactor52 for magnetic amplifier control winding 37 are serially connectedthrough normally open contacts 56C across the direct voltage supplyconductors 33 and 39. Similarly, the operating coil 58a of relay 58 andthe operating coil 51a of reversing contactor 51 for magnetic amplifiercontrol winding 42 are serial-ly connected through normally opencontacts 55e across the direct voltage supply conductors. Thus the relay55 which responds to the output voltage of magnetic amplifier 27ultimately controls the reversing contactor 51 for the control winding42 of magnetic amplifier 40 and also the reversing contactors 13 and 14for motor primary winding 6. Similarly, relay 56 which is responsive tothe output voltage of magnetic amplifier 40 ultimately controls thereversing contactor 52 for the saturation control winding of magneticamplifier 27 and also the reversing contactors 11 and 12 for motorprimary winding 5.

For the purpose of initially energizing and deenergizing the system, apush button switch accessory and a run contactor controlled thereby areprovided. The accessory is illustrated as comprising start and stop pushbutton switches 59 and 60 having normally open and normally closedcontacts respectively, serially connected in, circuit with the operatingcoil 61a of a run contactor 61.

With the foregoing understanding of the elements and their organization,the operation of the invention will readily be understood from thefollowing detailed description. It is assumed that the leaf of thebridge is in its fully seated position in which it is illustrated. Withthe master switch 44 in its central off position all relays andcontactors are deenergized and their contacts are in their normalpositions in which they are illustrated.

The control is energized by depressing the start switch to close itscontacts and complete an energizing circuit for the operating coil 61aof run contactor 61 through normally closed contacts of the stop switch.In response to energization the run contactor picks up and seals itselfin through normally open contacts 61h in parallel with the contacts ofthe start switch 59 which may now be released. The operating coil 11a offorward contactor 11 for primary winding 5 is now connected acrossalternating supply conductors 33 and 34 through the normally closedinterlock contacts 12e of reverse contactor 12 and normally closedcontacts 5711 of relay 57 to bus 33a and thence through contacts 61h ofrun contactor 61 in the closed position thereof. Similarly the operatingcoil 14a of reverse contactor 14 for primary winding 6 is energized'through normally closed interlock contacts 13e of forward contactor 13and normally closed contacts 58b of relay 58. Responsively toenergization, the main contacts of forward contactor 11 are closed toconnect primary winding through saturable reactors 15, 16 and 17 to thethree-phase supply conductors 8, 9 and 10 for forward rotation. Likewisethe main contacts of reverse contactor 14 are closed to connect primarywinding 6 to supply source through saturable reactors 18, 19 and 20 forreverse rotation. As long as forward contactor 11 is picked up, itsnormally closed interlock contacts 11f in the circuit of operating coil12a of the reverse contactor 12 are open and prevents its energization.Similarly, when reverse contactor 14 is energized and closed itsnormally closed interlock contacts in the circuit of coil 13a of forwardcontactor 13 are open and prevent its energization as long as reversecontactor 14 is picked up.

The negative biases produced by the bias windings 36 and 41,0f magneticamplifier 27 and 40 respectively reduce the output voltage of bothamplifiers to approximately of maximum. Consequently, saturable reactors15, 16, 17, 18, 19 and 20 are relatively unsaturable and present highimpedance in the circuits of primary windings 5 and 6.

As a net result of the foreging initial operating conditions, primarywinding 5 produces a very low forward torque'and primary winding 6produces an equal reverse torque. Consequently the net torque of themotor is zero and it is at standstill.

Movement of the master switch 44 to its first forward position connectsoutput terminal 44a to a point on potentiometer 45 that is slightlypositive with respect to the center terminal 45a. Simultaneously itopens normally closed contact 44b. Thus a reference voltage ofrelatively low value appears across output terminals 44a and 45a and issupplied to the comparison circuit 48. As a result, main saturationcontrol winding 37 of magnetic amplifier 27 is energized in opposingrelationship with respect to the negative bias coil 36 and mainsaturation control winding 42 of magnetic amplifier 40 is energized inadditive relationship with respect to its negative bias coil 41.Consequently the output voltage of magnetic amplifier 27 is increasedthereby to increase the saturation of saturable reactors 15, 16 and 17and correspondingly reduce their reactances. Conversely the outputvoltage of the magnetic amplifier 40 is reduced substantially to zerothereby to increase the reactances of reactors 18, 19 and 20. Thedecreased reactance of reactors 15, 16 and 17 increases slightly theforward torque of primary winding 5 and increase reactances of reactors18, 19 and 20 decreases substantially to zero the reverse torque ofprimary winding 6. The result is that the motor has a net forward torqueof sufficient value to take up the backlash in the gearing butinsufficient to accelerate the leaf 3.

On the second point forward of the master switch the output terminal 44ais connected to a point on potentiometer 45 of still more positivevoltage thereby to increase the reference voltage to a higher value.This circulates more current through the saturation control windings 37and 42 of magnetic amplifiers 27 and 40 thereby to increase the outputvoltage of amplifier 27 and correspondingly to increase the saturationof reactors 15, 16 and 17 and the torque produced by primary winding 5.The torque of primary winding 6 remains substantially unaltered andconsequently the net torque of the motor increases to a value at whichthe motor starts and accelerates the leaf 3. As the motor speed risesthe tachometer 43 generates a voltage which increases correspondingly.This speed voltage is supplied to the comparison circuit in opposingrelation with respect to the reference voltage. The accelerationcontinues until a balanced condition between reference voltage and speedvoltage is established.

In the event of an ice load on the leaf so heavy as to preventacceleration of the motor the tachometer generator 43 does not supplyany opposing voltage to the comparison circuit and the currents flowingin the saturation control winding 37 and 42 increase in value. Thisincreases the output voltage of magnetic amplifier 27 to approximately50% of maximum. In response to this increased output voltage relay 55picks up and closes its normally open contacts 55C to completeenergizing circuits for the operating coils 51a and 58a of reversingcontactor 51 and relay 58. It also opens its normally closed interlockcontacts 55b in circuit with operating coil 56a of voltage-sensitiverelay 56.

Responsively to energization contactor 51 picks up to open its normallyclosed contacts 51b and 51e and to close its normally opened contacts51d and 51e thereby to reverse the polarity of saturation controlwinding 42 s0 that it opposes the negative bias coil 41. Since it is inseries relationship with saturation control coil 37 the output voltageof magnetic amplifier 40 becomes equal to the output voltage of magneticamplifier 27, i.e. approximately 50% of maximum. Owing to the prioropening of contacts 55b in its coil circuit relay 56 cannot respond tothis increased output voltage yof magnetic amplifier 40 and accordinglyremains deenergized. The increased output voltage of amplifier 40increases the saturation of reactors 18, 19 and 20 thereby to increasethe voltage supplied to primary winding 6 and to increase its torque tosubstantially the same value as the torque produced by winding 5.Simultaneously, relay 58, in respouse to energization, opens itsnormally closed contacts 58b and closes its normally opened contacts 58ethereby deenergizing and dropping out reverse contactor 14 and pickingup forward contactor 13. The opening of contactor 14 and the closing ofcontactor 13 reverses the phase rotation of the primary winding 6 andthereby reverses the direction of its torque so that both primarywindings 5 and 6 contribute relatively large substantially equal amountsof motoring torque in the forward direction to raise the leaf 3.

Operation of the master switch 44 to successively higher speed positionseffects operation of the motor at correspondingly higher speeds.

Another operating condition frequently encountered is wind in adirection either to open `or close the leaf. Assume in this case a windin a direction tending to open the leaf. In this event the wind assiststhe motor and causes its speed to rise above the speed set by the masterswitch. This causes the voltage of the tachometer generator to increaseand thereby to reduce the current in the main saturation controlwindings 37 and 42 of the magnetic amplifier 27 and 40. The net effectof this is substantially to reduce the forward motoring torques producedby both primary windings 5 and 6.

If the wind is'so strong that the bridge continues to accelerate, theoutput voltages of both magnetic arnplifiers continue to decrease andreduce the forward motoring torque of both primary windings 5 and 6. Asthe' output voltage of amplifier 27 falls below the drop out value ofvoltage-sensitive relay 55, it drops out to open its normally closedcontacts` 55a to deenergize and drop out reversing contactor '51' andrelay 58. In its dropped out position reversing contactor 51 reversesthe connections of main saturation control winding 42 so that it aidsthe negative bias coil 41 to reduce the output voltage of amplifier 40substantially to zero which in turn reduces the forward motoring torqueof primary winding 6 to a very low value. Relay 58 in dropping outcloses its normally closed contacts 58h and opens its normally opencontacts 58C thereby to cause reversing contactors 13 vand 14 to reversethe connections of the primary winding 6 to the source so that itstorque opposes the torque of primary winding 5. Thus the net forwardmotoring torque of the motor is still further reduced.

In the event that the wind is so strong that it continues to acceleratethe leaf until the tachometer voltage exceeds the reference voltage, thecurrent in the comparison circuit reverses and the main saturationcontrol coil 42 opposes the negative bias coil 41 and main saturationcontrol winding 37 aids negative bias coil 36. This decreases the outputvoltage of amplifier 4t) the net result of which is to decrease theforward motoring torque of primary winding 5 and to increase the reversemotoring torque of primary winding 6. Under the assumed conditions, thetachometer voltage is increasing and continues to increase the currentin the main saturation control windings 37 and 42 to whatever extent maybe necessary even to the extent that the reverse motoring torque ofprimary Winding 6 becomes greater' than the forward m0- toring torque ofprimary winding 5. This results in a soft plugging of the motor whichcauses it to exert a net braking torque.

If the wind causes the speed of the leaf to become appreciably` greaterthan the reference, the output voltage of magnetic amplier 40 willexceed the pickup value of voltage-sensing relay 56. 56 opens itsnormally closed interlock contacts 5611 and closes its normally opencontacts 56C to complete an energizing circuit for the operating coils52a of reversing contacter 52 and 57a of relay 57. Responsively toenergization, reversing contacter 52 picks up and reverses theconnections in the comparison circuit of main saturation control winding37 so that it opposes the negative bias coil 36 and increases the outputvoltage of magnetic amplifier 27 in response to increasing current inthe comparison circuit. gization, relay 57 picks up and opens itsnormally closed contacts 5711 to deenergize and drop out the forwardreversing contactor 11 for primary winding 5, and closes its normallyopen contacts 57C to energize and pick up reverse contacter 12. Thisconnects the primary winding 5 to the source for reverse rotation sothat the motor is now completely plugged with respect to both primarywindings and `exerts maximum braking torque which eiiectivelycounteracts the heavy wind load.

Limit switches (not shown) take the control away from the master switchat the nearly open and nearly seated positions of the leaf. The controlis regained by moving the master switch -to the oil or first position ineither direction.

Although in accordance with the provisions of the patent statutes, thisinvention is illustrated as embodied in concrete form and the principlethereof has been explained together with the best mode in which it isnow contemplated. Applying that principle it will be understood that theapparatus shown and described is merely illustrative and that theinvention is not limited thereto since alterations and modificationswill readily suggest themselves to persons skilled in the art withoutdeparting from the true spirit of the invention or from the scope of theannexed claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

l. A motor control system comprising in combination an induction motorhaving dual primary windings, current limiting means and reversingswitching means connected in circuit with each of said windings, amaster switching device and means controlled thereby for establishing areference voltage representative of a desired speed of said motor, meansfor deriving from said motor a control voltage indicative of its speed,and means responsive to the difference of said voltages for controllingsaid current limiting means and said reversing switching means toconform the actual speed of said motor with said desired speed.

2. A motor control system comprising in combination an induction motorhaving dual primary windings, reversing switching means connected incircuit with each of said windings, a master switching device and meanscontrolled thereby for establishing a reference voltage In picking up,the relay Simultaneously, in response to eneri representative of adesired speed of said motor, means for deriving from said motor acontrol voltage indicative of `its speed, and means responsive to thedifference of said voltages for controlling said reversing switchingmeans to control the relative directions of the torques produced by saidprimary windings.

3. A motor control system comprising in combination an induction motorhaving dual primary windings, current limiting means and reversingswitching means connected in circuit with each of said windings, amaster switching device and means controlled thereby for. establishing areference voltage representative of a desired speed of said motor, meansfor deriving from said motor a control voltage indicative of its speed,and means responsive to the dilerence of said voltages for jointlycontrolling said reversing switching means andv current limiting meansto control the relative directions andV relative magnitudes of thetorques produced by said primary windings.

4. A motor control system comprising in combination, an induction motorhaving dual primary windings, a separate series combination connected incircuit with each of said windings, each comprising a reversingswitching means and a current limiting means, a master switching devicefor establishing a reference voltage representative of the desired speedof said motor, means for deriving from said motor and said referencevoltage an error voltage indicative of the difference between areference speed and the actual speed of said motor, means responsive tosaid error voltage for controlling said current limiting means to varythe relative torques produced by said primary windings comprisingmagnetic amplifier means having a saturation control winding and havingits output connected to control said current limiting means and itssaturation control winding connected to be responsive to said errorvoltage, reversing switching means for said saturation control windings,and means responsive to the output voltage of said magnetic amplifiermeans for jointly controlling said reversing switching means for saidprimary windings and said reversing switching means for said magneticamplifier means.

5. In combination an induction motor having dual primary windings, aseries combination connected in circuit with each of said windingscomprising a reversing switchng means and a current limiting saturablereactor provided with a saturation control winding, a master switchingdevice for varying the current limiting eifect of said reactor, meansfor deriving from said motor and said master switching device an errorvoltage indicative of the difference between -the speed of said motorand a reference speed established by said master switching device, andmeans responsive to said error voltage for selectively controlling theoperation of said reversingy switching means to control the relativedirections of the torques produced by said primary windings.

6. In combination an induction motor having dual primary windings, aseries combination connected lin circuit with each ot said windingscomprising a reversing switching means and a current limiting saturablereactor pro vided with a saturation control winding, a master switchingdevice, means for deriving from said motor and said master switch anerror voltage indicativey of the diierence between a reference speedestablished by said master switch the speed of said motor, and magneticamplilier means responsive to said error voltage for selectivelycontrolling the operation of said reversing switching means to controlthe relative directions of the torques of said primary windings and forvarying the current limiting effects of said reactors to vary themagnitudes of said torques.

7. In combination an induction motor having dual primary windings, aseries combination connected in circuit with each of said windingscomprising a reversing switching means and a current limiting saturablereactor provided with a saturation control winding, a master switchingdevice for providing a reference voltage representative of the desiredspeed of said motor, means for deriving from said motor a voltagerepresentative of its speed, a voltage comparison-circuit for providingan error voltage representative of the diiference of said referencevoltage and said speed voltage, and magnetic amplifier means having acontrol winding included in said comparison circuit and having outputwindings connected to the saturation control windings of said reactorsfor controlling their current limiting eiects to vary the torques ofsaid primary windings, and means responsive to the output of saidampliers for controlling the operation of said reversing switching meansto control the relative directions of said torques.

8. A motor control system comprising in combination, an induction motorhaving dual primary windings, a series combination connected in circuitwith each of said windings comprising a reversing switching means and acurrent limiting satura'ble reactor provided with a saturation controlwinding, a master switching device for providing a reference voltagerepresentative of the desired speed of said motor, a tachometergenerator driven by said motor for generating a voltage representativeof its speed, a voltage-comparison circuit for providing an errorvoltage representative of the dilerence of said reference voltage andsaid speed voltage, and magnetic amplifier means having control windingsincluded in said comparison circuit and having an output connected tothe saturation control windings of said reactors for controlling theircurrent limiting eifects, and voltage-sensitive relay means 10responsive to the output of said amplifiers for controlling theoperation of said reversing switching means to control the relativedirections of the torques produced by said primary windings.

9. A motor control system comprising in combination, an induction motorhaving dual primary windings, a series combination connected in circuitwith each of said windings comprising a reversing switching means and acurrent limiting saturable reactor provided with a saturation controlwinding, a master switching device for providing a reference voltagerepresentative of the desired speed of said motor, means for derivingfrom said motor a voltage representative of its speed, avoltagecornparison circuit for providing an error voltage representativeof the difference of said reference voltage and said speed voltage, apair of magnetic amplifiers each having a bias winding and each having acontrol winding connected in series with the control winding of theother in said comparison circuit, and each having an output connected tothe control winding of a different one of said reactors for controllingits current limiting effect, second reversing switching means for saidcontrol windings, and voltage-sensitive means responsive to the outputVoltage of said magnetic amplifiers for controlling said primary windingreversing switching means to control the relative directions of thetorque produced by said primary windings and for controlling said secondreversing switching means for controlling the relative magnitudes ofsaid torques.

No references cited.

